1. Field of the Invention
The present invention relates in general to a scanner having a top cover and an optical module that maintain the same distance from each other during the scanning process.
2. Description of the Related Art
A scanner is an example of the application of lenses to form images. FIG. 1 illustrates the imaging principle of a scanner, in which reference notation "A" represents a surface of a document, "11" represents a lens and "A'" represents a surface of an optical sensor on which an image of the document is formed. How the two surfaces A and A' are parallel to each other (i.e., the parallelism of the surface A with respect to the surface A') greatly influences the resultant image quality. Particularly, a high-resolution scanner requires the precise parallelism of the surface A with respect to the surface A', wherein deviation of the parallelism is restricted less than 1 mm or the uniformity of the image will be poor.
In practical application, the surface A of a scanner system can alternatively indicate a glass surface of a top cover of a scanner on which the document is disposed. A' is the surface of a charge coupled device (CCD). Between the two surfaces A and A' are lenses. In some situations, for example, in order to shorten the light path, additional reflective mirrors are provided in proper positions between the two surfaces A and A'. Furthermore, the lenses and the charge-coupled device are generally built together in a module. Hereinafter, such a module including the lenses and charge coupled device is named the "Optical Module". The surface of the glass of the top cover and the surface of the optical module must be accurately parallel to each other so as to obtain a good image quality. So far, at least three kinds of scanners provide solutions regarding this issue:
(1) FIGS. 2A-2C depict the first kind of conventional scanner, in which an optical module 21 is mounted in a bottom cover 24 via a rod 22 and a wheel 23 (FIGS. 2A and 2B). Then, the bottom cover 24 are assembled together with a top cover 27 of the scanner with two spacers 25', 26' provided therebetween (FIG. 2C). In this case, both the top cover 27 and the optical module are supposed to be parallel to the bottom cover 24. PA1 (2) FIGS. 3A-3C depict the second kind of conventional scanner, in which two iron pieces 32, 33 of the same height are fixed on a bottom cover 31 (FIG. 3A). An optical module 34 is mounted therein (FIG. 3B), wherein one end of the optical module 34 is put around a shaft 33' of the iron piece 33, and the other end of the optical module 34 rests on the iron piece 32. The bottom cover 31 is assembled together with a top cover 35 of the scanner, with the top cover 35 supported by the iron pieces 32, 33 (FIG. 3C). In this case, the two iron pieces 32, 33 are of the same height so that the top cover 35 is parallel to the optical module 34. PA1 (3) FIGS. 4A-4B depict the third kind of conventional scanner, in which an optical module 42 is mounted in a bottom cover 41 from which supporting posts 45, 46 of the same height extend (FIG. 4A). The bottom cover 41 is assembled together with a top cover 43 of the scanner (FIG. 4B), with the glass 44 of the top cover 43 supported by the supporting posts 45, 46. In this case, the two supporting posts 45, 46 are of the same height so that the glass 44 of the top cover 43 is parallel to the optical module 42.
In the first and third cases, the optical modules and top covers are mounted on the bottom covers, which are generally injection molding pieces produced by the injection molding method. However, controlling the deformations of injection molding pieces during the production process is not easy. That is, controlling the tolerances of injection molding pieces is difficult. Therefore, using the bottom covers function as the bases of the optical module and the top cover in order to obtain parallelism of the optical module with respect to the top cover easily fails. Furthermore, the optical module is supported by a wheel. In practice, wheels cannot be exactly round. An optical module supported by a wheel shakes slightly when the wheel is rotated during the scanning process.
In the above-mentioned second case, the parallelism of the optical module with respect to the top cover is mainly determined by the bent angles of the iron pieces. Therefore, shaping the iron pieces by bending (i.e. using the so-called "Sheet-Metal Forming Method") must be accurately controlled. However, using the Sheet-Metal Forming Method will lead the iron pieces into springback. Accurately compensating the springback is very difficult. Besides, a scanner provided with iron pieces is heavy and expensive.